Excimer lasers are currently used for integrated circuit lithography as a light source. They illuminate a mask whose image is then projected on a silicon wafer. Due to the limitations set by optical diffraction, the smaller the desired printed features, the shorter the wavelength of the light must be.
Currently, a KrF excimer laser working at 248 nm is in worldwide use in chip manufacturing with a features size as small as about 0.25.mu.. ArF excimer lasers are entering the market as a light source for even smaller feature microlithography, as small as about 0.15.mu..
Due to the very limited number of materials which are transparent to 193 nm light, the imaging lens for 193 nm exposure tools will not have good chromatic correction. Therefore, it is important to provide a laser with a very narrow spectrum of less than 1.0 pm. The spectrum of the free-running ArF laser is about 200 pm. Therefore, significant spectral line narrowing should be done in order to be able to use the laser in refractive optical microlithography.
Techniques for decreasing the bandwidth of laser output beams are well known. Several such techniques used on excimer lasers are discussed by John F. Reintjes on pages 44-50 in Laser Handbook, Vol. 5, North-Holland Physics Publishing, Elsevier Science Publishers B.V. These techniques include the utilization of gratings including echelle gratings for wavelength selection. The use of beam expanding prisms ahead of the grating can increase the effectiveness of the grating.
A typical prior art ArF excimer laser is shown in FIG. 1. The resonant cavity of the excimer laser 2 is formed by an output coupler 4 (which can be a 30 percent partially reflecting mirror) and an echelle grating 16. A portion of the laser beam transmits through the output coupler and exits the laser as an output beam 18. The remaining part of the beam is reflected by the output coupler 4 back to the chamber 3 for amplification and narrow banding. Beam 20 (having a cross section of about 3 mm in horizontal direction and about 15 mm in the vertical direction) exits the rear of the laser chamber 3. This portion of the beam is expanded in the horizontal direction by prisms 8, 10, and 12 and reflected by a mirror 14 onto the echelle grating 16. Grating 16 is arranged in Littrow configuration, so that a selected narrow band of wavelengths is reflected back off mirror 14 and back through prisms 12, 10, and 8 and into the chamber 3 for further amplification. Light at wavelengths outside the selected narrow band is angularly dispersed so that the out-of-band light is not reflected back into the laser chamber. The absolute values of the desired spectral wavelength range is selected by pivoting mirror 14. Total beam expansion of the prior art ArF excimer laser is about 20.times.. The beam has a horizontal polarization (p-polarization for the prisms). Typical ArF lasers operating in a pulse mode may have a cavity length of about 1 m and produce pulses having a duration of about 20 to 30 ns. Thus, photons within the resonance cavity will make, on the average, about 3 to 5 round trips within the cavity. On each round trip, about 70 percent of the beam exits at the output coupler and about 30 percent is sent back for further line narrowing. The beam is repeatedly line narrowed as it passes through the line-narrowing module.
The spectrum of excimer lasers used for lithography is generally specified in two different ways. One is the spectral width measured at half-maximum level (FWHM). The other spectral specification commonly used to characterize microlithography excimer lasers is referred to as the "95% integral". This is the spectral width of the portion of the pulse containing 95% of the total pulse energy. Typically, FWHM values of about 0.6 pm and 95% integral values of about 1.5 pm are required for 193 nm lithography.
The prior art ArF excimer laser referred to above is capable of meeting the above mentioned spectral requirements and it has been used in the prototype stages of ArF lithography development. However, there are still significant problems associated with the use of the prior art ArF laser in production scale lithography applications. One of the most significant problems is caused by loss of light reflected by the prisms surfaces.
The conventional way to reduce reflections from optical surfaces is to use various anti-reflection (AR) coatings. It is relatively easy to make an anti-reflection dielectric coating for light at normal incidence. Such a prior art coating would have alternating quarter-wave layers of high refractive index material and low-refractive index material, designed in such a way, that the light waves, reflected from these layers undergo destructive interference with each other, so that the total reflection is minimized. Currently, a number of suppliers offer anti-reflection coatings which have total losses due to reflection and absorption of less than 1%. For example, Acton Research Corporation (Acton, Mass.) offers such a coating.
Unfortunately, the 20.times. times magnification requirement using three prisms means that the light incidence angle on the hypotenuse of the prism needs to be about 74 degrees. It is very difficult to make an AR coating at 193 nm for such a large incidence angle. At this angle, the reflection reduction efficiency of the stack of quarter-wave layers is reduced dramatically, so that the required number of layers is substantially increased and the thickness and density tolerance on each layer is greatly tightened. In addition, the choice of materials with good transmission at such a short wavelength is greatly limited, and many of them are either hygroscopic or too soft. What is needed for a production scale ArF laser however, is a coating which has a reflection less than about 2.0% at 193 nm, absorption less than 0.5%, the ability to withstand several billion pulses at 193 nm with a pulse energy density of up to 20 mJ/cm.sup.2, and good compatability with industrial environments. It is extremely difficult and expensive to make such a coating, which means it is not currently available for large scale production at an acceptable cost.
Therefore, the only alternative currently available--is not to use any hypotenuse coatings at all. The result is substantial losses due to Fresnel reflections off of the prism hypotenuse surfaces. In this case, the reflectivity of each surface is about 9% for CaF.sub.2 prisms and about 8.6% for fused silica prisms. Considering, that the light goes through each prism twice per round trip, we have total of 6 reflections at hypotenuse surfaces with the total losses from these reflections equal to 43.2% for CaF.sub.2 and 41.7% for fused silica. The impact of these losses is a significant reduction in laser efficiency. For example, the laser, which has an output of 5W at 1 kHz when using all AR coated prisms will produce about 3W when no hypotenuse AR coating is used. Such a reduction of efficiency means reduced product throughput, reduced operating lifetime of the laser and increased cost of ownership.
Therefore, what is needed, is an alternative technique, which permits the required line narrowing without substantially reducing the laser efficiency, and which is suitable for large-scale production requirements.